If the StackPanel has more elements than can be displayed on the screen (or in whatever container the StackPanel happens to find itself), the elements towards the bottom (or right) won’t be displayed.
If you fear that the phone’s screen is not large enough to fit all the children of your StackPanel, you can put the StackPanel in a ScrollViewer, a control that determines how large its content needs to be, and provides a scrollbar or two.
Actually, on Windows Phone 7, the scrollbars are more virtual than real. You don’t actually scroll the ScrollViewer with the scrollbars. You use your fingers instead. Still, it’s convenient to refer to scrollbars, so I will continue to do so.
By default, the vertical scrollbar is visible ...